[Info-vax] SET TERM /TTSYNC on ancient VMS versions

[hans.huebner at gmail.com]

Let me try to sum this up:

Software flow control can be enabled and disabled in the terminal driver (SET TERMINAL /HOSTSYNC and /TTSYNC).
When software flow control is switched off, the terminal driver does not interpret XON and XOFF (/TTSYNC) and also does not send XON and XOFF (/HOSTSYNC).  No other flow control mechanism is offered by VMS on asynchronous serial connections.
Some asynchronous serial mulitplexers offered hardware flow control mechanisms (proprietary or as standardized in TIA-232-E/F), but those mechanisms were not controlled by or visible in VMS.

The terminal driver is used not only for asynchronous serial lines, but also for virtual terminal connections.  For such connections, many of the terminal driver options don't make that much sense (e.g. SET TERM/MODEM on a telnet connection).  It was possible that the virtual terminal protocol forwarded hardware related settings to the asynchronous serial controller, if one was used.  I think LAT did that, but I could be wrong.

I am thus concluding that binding of Ctrl-S and Ctrl-Q in VAX LISP was not disallowed because of a "System restriction", but rather because it would be dangerous to do so on serial terminals of the time because terminals themselves would send those codes for flow control purposes.

Knowing this allowed me to proceed and patch VAX LISP so that it allows me to bind those keys (https://netzhansa.blogspot.com/2018/08/using-vms-patch-utility-to-fix-vax-lisp.html).  I will not call DEC support if it does not work, but as I've read in the PATCH manual, my warranty is void anyway because I have patched an executable supplied by DEC. :)